1. Field of the Invention
The present invention relates to a golf club having a structure by which a speed of the head of the golf club is improved.
2. Description of the Related Art
A flight distance of a golf ball hit by a golf club varies in accordance with the rebounding property of the golf ball. It is commonly thought that the golf ball flys a longer distance if the golf ball has a larger rebounding force and the face of the head of the golf club is ever harder. Nevertheless, a report has been made stressing that it is not always satisfactory to merely harden the hitting face of the head of the golf club to an infinite hardness, but instead an appropriate hardness should be selected for the head hitting face. For example, Japanese Unexamined Patent Publication (Kokai) No. 61-22875 discloses that there is an optimum hardness of the hitting face of the head of the golf club, which should be determined based on a mechanical impedance, since a rebounding property of the golf ball is reduced if a hardness of the hitting face of the head of the golf club exceeds the optimum hardness value. In addition to the consideration of these quasistatic characteristics, dynamic characteristics are dominant when considering a flight distance of a golf ball.
A flight distance of a golf ball substantially depends on a speed of a head of a swung golf club, a weight of the head, and a take off angle of the golf ball. To increase the speed of the head, it is necessary to increase the speed of the swing of the golf club, but the speed of the head will not be increased as much as expected if the player swings the golf club as if it is a stiff rod. Namely, it is necessary to swing the golf club with a full utilization of an elasticity of the shaft, in order to increase the speed of the head; i.e., if the golf club is swung such that the shaft is elastically deformed at an initial stage of a downswing so that the head lags behind the shaft, and this elastic deformation is restored just before an impact of the head with the golf ball, then the speed of the head due to the elastic restoration function of the shaft is added to the speed of the swing, and thus the speed of the head is increased. From this viewpoint, the elasticity of the shaft is very important, and recently, a swing theory related to a stiffness of the shaft or a frequency theory combining a stiffness of the shaft with a weight of the head has been developed.
Nevertheless, there have been no proposals to increase the speed of the head by the utilization of an impact characteristic derived from a torsional bending of the shaft during the impact. The inventor of the present application found that a portion of the shaft near the distal end thereof is dynamically bent in the form of an obtuse U for a short time, as illustrated in FIG. 4B of the attached drawings by the arrow X, due to an impact of the head with the golf ball. The impact lasts for only a very short time, and usually such a bending of the shaft due to an impact of the head with the golf ball cannot be observed. Therefore, there have been no conventional concepts of increasing the speed of the head and stabilizing a flight direction of the golf ball by taking into consideration the impact of the head with the golf ball.
The inventor of the present application visualized the deformation of the distal end portion of the shaft and found that the distal end portion and a portion of the shaft near the neck of the head are dynamically bent, and concluded that such a dynamic bending of the shaft due to an impact of the head with the golf ball is important in consideration of an increase of the speed of the head of the golf club. In brief, when the head comes into contact with the golf ball, it is displaced backward relative to the shaft for a short time, and accordingly, the shaft is subjected at the proximate end thereof to a force of the swing which causes the shaft to continue to advance, and at the distal end thereof, is subjected to a force of the head which causes the shaft to be displaced backward, resulting in a bending of the distal end portion of the shaft. Namely, while the entire golf club has an advancing inertia during the swing, the head only is subjected to a backward force due to the impact and a portion of the shaft is instantaneously deformed thereby, and in this instant, an elastic strain energy is accumulated in the shaft. Namely, the head is displaced backward relative to the shaft by contact with the golf ball, and since the swing is continued, the head with the shaft advances relative to the golf ball and remains in contact with the golf ball after the first contact of the head with the golf ball. Thereafter, the golf ball rebounds from the head a certain time after the first contact therebetween. In this specification, the time from a point at which the golf ball is first in contact with the head to a point at which the golf ball rebounds from the head is referred to as "a contact time Tc". It has been found that, in conventional golf clubs, the initial bending of the distal end portion of the shaft due to the impact of the head with the golf ball is restored after this contact time Tc has elapsed. Therefore, the head is lags behind the shaft, and this is one reason for reducing the speed of the head.